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Weathering rates derived from field studies and laboratory experiments with various aquatic solvents
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION I N T E R N A T I O N A L SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.
311
2nd
W E A T H E R I N G RATES DERIVED F R O M FIELD STUDIES AND LABORATORY E X P E R I M E N T S W I T H VARIOUS A Q U A T I C SOLVENTS
MATTHESS G. and SCHENK D. Geological and Paleontological Institute, University of Kiel, FRG.
METHODS Three experimental setups (batch, fluidized bed reactor and column experiments) have been used to determine the dissolution behaviour of silicate minerals. The aquatic solvents used are modified by organic ligands and/or by a premineralisation (SCHENK et al., 1989). The presence of organic and inorganic solutes are typical for natural solutions in weathering profiles. This is simulated by adding citric, tartaric, salicilic and pyrocatechol violet, or by CsC1. The experiments use feldspars, mica, SiO2 modification and natural glaciofluvial sands as solids phases. The results from laboratory experiments are compared with field observations. I N F L U E N C E OF P R E M I N E R A L I S A T I O N AND O R G A N I C LIGANDS The initial and final stages of albite dissolution in batch experiments with CsCl-solutions show a decrease of the Si dissolution rate and in a smaller final Si-concentration compared with experiments with deionized water. The experiments with pH 6 are more sensitive to Cs + than the experiments with pH 4.5, where the ratio of H + to Cs ÷ is much higher than at pH 6. The dissolution rates depend significantly on the pH value between pH 2,3 and pH 8, with a minimum dissolution rate at pH 6.0. The scattering of the rates between pH 2.3 and pH 2.5 is evidently due to the type of solid species (with highest rates for albite and lowest for orthoclase). Whereas at the low pH-values mentioned above, no significant effect of the type of organic ligands are observed, at pH 4.5 and pH 6 the mineral species has a lower effect on the dissolution rates than the type of organic ligands present in the solution. In pH-fixed dissolution experiments with pyrocatechol violet the dissolution process ends at pH 4.5 after 500 h and at pH 6.0 after 400 h reaction
time. It is not possible to restart the dissolution process by cleaning the feldpar surface with HC1, but heating of rock material up to 500 °C apparently destroys the mineral-organic complex so that the dissolution process may continue. The application of complex-forming organic ligands leads to significantly higher concentrations of dissolved A1. In the range between pH 4.5 and pH 6, which are fixed by maintaining specific concentrations of the respective organic ligands the precipitation of aluminium species is prevented by complexation so that the AI/Si ratio in the solution is equal to that in the feldspar. The apparent incongruency of the dissolution process in deionized water may therefore be explained by a specific precipitation of the leached A1. In the fluidized bed reactor experiments the pH-value were abruptly lowered from 6.5 to 4. This pH-drop causes for a short period a strong increase of the concentrations of all solutes, caused by the fast dissolution of newly formed solid secondary phases. The concentration of dissolved A1 in the columns reflects also the transport velocity of free organic ligands. The results support the approach that the dissolution processes in a natural weathering profile are also controlled by the sorption characteristics of the organic ligand and consequently by their retardation. W E A T H E R I N G RATES DERIVED F R O M F I E L D STUDIES The weathering rates derived from the dissolved sodium content of the pore water originates from the input by precipitation and corrected for input by precipitation gives as a mean value for the whole profile a rate in the range of 10 -17 mole Na+.g -1 s -1. ALBERTSEN et al., (1977) calculated sodium
312
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.
2nd
depletion rates for catchment in the Segeberger Forst with input/output comparisons to be in range of 2.5.10 -15 mole Na+.g -rs -1, As the dissolution rates in the laboratory experiments are calculated from Si concentrations a direct comparison with Si dissolution rates from field studies would be of interest. However, Si concenration in the pore water are controlled by the solubility of both amorphous SiO2 and quartz. Only above 1.5 m the pore waters are undersaturated with respect to these species and the observed gradient enables a calculation of the silica release rates, which however must be regarded with caution. The A1 concentration is also controlled by reversible dissolution-precipitation reactions with amorphous alumina or aluminium silicates. Positive mass balances for the given unit volumes can only be used for calculating dissolution rates when the A1 concentration under the given pH-value is far away from saturation or precipitation of A1 is prevented by complexation with organic ligands. This apparently holds only for close to earth surface pore solutions. According to thermodynamic calculations 80-95 % of the dissolved aluminium should be complexed by organic ligands. The relatively unstable polymeric organo complexes are destroyed very fast if the supply of free organic ligands decreases with increasing distance from the soil in which the organic substances are mainly produced. This can be seen in the decrease of the aluminium concentration from 4 m below surface. The so calculated dissolution rate from the aluminum concentrations in the pore solutions down to 2.8 m below surface (SEG 3) (1.3.10 -15 mole
Al.s-l.g -1) differs only slightly from those derived from silica concentrations. This indicates a nearly congruent dissolution process which is consistent to laboratory results. Weathering rates based on A1 concentrations can be calculated futhermore by the analyses of the binding forms. The input of A1 into the system by rainfall is neglectable. The outflow from the profile below the region with a notable Corg-content can not be larger than the average velocity rate times the maximum solubility under the given pH-value. Orders of magnitude more A1 remains in the system as secondary Al-phases, which can be detected by the binding form analyses and normalised interpolated to totals amounts per volume element. A rate of 4.9.10 -16 mole Al.g -1. s -1 is calculated on an assumption for the duration of the elapsed weathering time. According to the results of modell calculations a total weathering time of 10000 years seems to be realistic and is consistent with local geological history. During this time the upper part of the profile was decalcificed which has lowered the pH, has increased the weathering rates as well as the outflow due to increasing A1 solubilities at pH values unbuffered by calcite equilibrium. The assumption, that the observed aluminium and silica release are derived only from the reactive surfaces of the feldspars neglects the presence of amorphous and microcrystalline silica-phases in the sander material. Their mineralogical properties are not sufficiently known. The dissolution rates determined from laboratory experiments (Tab. 1) are in a reasonably good agreement with those derived from mass
TABLE I : Weathering rates derived from laboratory experiments (* - 9°C otherwise 22°C).
material
pH
solution
initial final dissolution rate
(mole Si-g-l.s-1) albite albite albite albite albite albite
4.5 4.5 4.5 4.5 6.0 6.0
deion, water deion, water 2nd run citric acid pyrocatechol viol. citric acid pyrocatechol viol.
1.5-10 6.0-10 6.4-10 1.4-10 9.9-10 5.0-10
11 13 12 11 12 13
5.2-10 3.1-10 7.5-10 4.8-10 5.5-10 3.9-10
13 13 13 13 13 13
natural sand natural sand natural sand
4.5 6.0 4.5
deion, water (*) deion, water (*) fulvic acid (*)
4.5-10 12 1.6-10 12 9.9-10 12
1.2-10 t3 1.4-10 13 3.8-10 13
quartz
4.5
deion.water
3.9-10 12
3.1-10 13
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Alx en Provence, France.
balances of the Segeberger Forst considering the simplified assumptions in the evaluation and the uncertainties in measurement (DAHMKE, 1988, PETERSEN, 1988). The results from column experiments with organic ligands support the field observations that between pH 4 and 8 the dissolution behaviour and maximum solubility of aluminium in natural systems depends mainly on the physicochemical properties of the organic ligands. REFERENCES
ALBERTSEN, M., MATTHESS, G., PEKDEGER, A. & H.D. SCHULZ (1977) : Quantifizierung von Verwitterungsvorgaengen.- Geol. Rundschau, 69 : 532-545; Stuttgart.
313
DAHMKE, A. (1988) : Loesungskinetik von Feldspat-reichen Gesteinen und deren Bezug zu Verwitterung und Porenwasser-Chemie natuerlicher Sander-Sedimente.- Diss. Uni. Kiel : 168; Kiel. PETERSEN, A. (1988) : Laboruntersuchungen zum Einfluss organischer Komplexbildner auf die Kinetik der Feldspatverwitterung.- Diss. Uni. Kiel: 210; Kiel. SCHENK, D., PETERSEN, A. & G. MA'I~I'HESS (1989) : Acceleration and retardation of silicate weathering due to organic substances.- Proc. 6th int. Symp. on Water/RockInteraction, Malvern, LrK (MILES, ed.) : 605608; Rotterdam, Brooldield.
311
2nd
W E A T H E R I N G RATES DERIVED F R O M FIELD STUDIES AND LABORATORY E X P E R I M E N T S W I T H VARIOUS A Q U A T I C SOLVENTS
MATTHESS G. and SCHENK D. Geological and Paleontological Institute, University of Kiel, FRG.
METHODS Three experimental setups (batch, fluidized bed reactor and column experiments) have been used to determine the dissolution behaviour of silicate minerals. The aquatic solvents used are modified by organic ligands and/or by a premineralisation (SCHENK et al., 1989). The presence of organic and inorganic solutes are typical for natural solutions in weathering profiles. This is simulated by adding citric, tartaric, salicilic and pyrocatechol violet, or by CsC1. The experiments use feldspars, mica, SiO2 modification and natural glaciofluvial sands as solids phases. The results from laboratory experiments are compared with field observations. I N F L U E N C E OF P R E M I N E R A L I S A T I O N AND O R G A N I C LIGANDS The initial and final stages of albite dissolution in batch experiments with CsCl-solutions show a decrease of the Si dissolution rate and in a smaller final Si-concentration compared with experiments with deionized water. The experiments with pH 6 are more sensitive to Cs + than the experiments with pH 4.5, where the ratio of H + to Cs ÷ is much higher than at pH 6. The dissolution rates depend significantly on the pH value between pH 2,3 and pH 8, with a minimum dissolution rate at pH 6.0. The scattering of the rates between pH 2.3 and pH 2.5 is evidently due to the type of solid species (with highest rates for albite and lowest for orthoclase). Whereas at the low pH-values mentioned above, no significant effect of the type of organic ligands are observed, at pH 4.5 and pH 6 the mineral species has a lower effect on the dissolution rates than the type of organic ligands present in the solution. In pH-fixed dissolution experiments with pyrocatechol violet the dissolution process ends at pH 4.5 after 500 h and at pH 6.0 after 400 h reaction
time. It is not possible to restart the dissolution process by cleaning the feldpar surface with HC1, but heating of rock material up to 500 °C apparently destroys the mineral-organic complex so that the dissolution process may continue. The application of complex-forming organic ligands leads to significantly higher concentrations of dissolved A1. In the range between pH 4.5 and pH 6, which are fixed by maintaining specific concentrations of the respective organic ligands the precipitation of aluminium species is prevented by complexation so that the AI/Si ratio in the solution is equal to that in the feldspar. The apparent incongruency of the dissolution process in deionized water may therefore be explained by a specific precipitation of the leached A1. In the fluidized bed reactor experiments the pH-value were abruptly lowered from 6.5 to 4. This pH-drop causes for a short period a strong increase of the concentrations of all solutes, caused by the fast dissolution of newly formed solid secondary phases. The concentration of dissolved A1 in the columns reflects also the transport velocity of free organic ligands. The results support the approach that the dissolution processes in a natural weathering profile are also controlled by the sorption characteristics of the organic ligand and consequently by their retardation. W E A T H E R I N G RATES DERIVED F R O M F I E L D STUDIES The weathering rates derived from the dissolved sodium content of the pore water originates from the input by precipitation and corrected for input by precipitation gives as a mean value for the whole profile a rate in the range of 10 -17 mole Na+.g -1 s -1. ALBERTSEN et al., (1977) calculated sodium
312
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.
2nd
depletion rates for catchment in the Segeberger Forst with input/output comparisons to be in range of 2.5.10 -15 mole Na+.g -rs -1, As the dissolution rates in the laboratory experiments are calculated from Si concentrations a direct comparison with Si dissolution rates from field studies would be of interest. However, Si concenration in the pore water are controlled by the solubility of both amorphous SiO2 and quartz. Only above 1.5 m the pore waters are undersaturated with respect to these species and the observed gradient enables a calculation of the silica release rates, which however must be regarded with caution. The A1 concentration is also controlled by reversible dissolution-precipitation reactions with amorphous alumina or aluminium silicates. Positive mass balances for the given unit volumes can only be used for calculating dissolution rates when the A1 concentration under the given pH-value is far away from saturation or precipitation of A1 is prevented by complexation with organic ligands. This apparently holds only for close to earth surface pore solutions. According to thermodynamic calculations 80-95 % of the dissolved aluminium should be complexed by organic ligands. The relatively unstable polymeric organo complexes are destroyed very fast if the supply of free organic ligands decreases with increasing distance from the soil in which the organic substances are mainly produced. This can be seen in the decrease of the aluminium concentration from 4 m below surface. The so calculated dissolution rate from the aluminum concentrations in the pore solutions down to 2.8 m below surface (SEG 3) (1.3.10 -15 mole
Al.s-l.g -1) differs only slightly from those derived from silica concentrations. This indicates a nearly congruent dissolution process which is consistent to laboratory results. Weathering rates based on A1 concentrations can be calculated futhermore by the analyses of the binding forms. The input of A1 into the system by rainfall is neglectable. The outflow from the profile below the region with a notable Corg-content can not be larger than the average velocity rate times the maximum solubility under the given pH-value. Orders of magnitude more A1 remains in the system as secondary Al-phases, which can be detected by the binding form analyses and normalised interpolated to totals amounts per volume element. A rate of 4.9.10 -16 mole Al.g -1. s -1 is calculated on an assumption for the duration of the elapsed weathering time. According to the results of modell calculations a total weathering time of 10000 years seems to be realistic and is consistent with local geological history. During this time the upper part of the profile was decalcificed which has lowered the pH, has increased the weathering rates as well as the outflow due to increasing A1 solubilities at pH values unbuffered by calcite equilibrium. The assumption, that the observed aluminium and silica release are derived only from the reactive surfaces of the feldspars neglects the presence of amorphous and microcrystalline silica-phases in the sander material. Their mineralogical properties are not sufficiently known. The dissolution rates determined from laboratory experiments (Tab. 1) are in a reasonably good agreement with those derived from mass
TABLE I : Weathering rates derived from laboratory experiments (* - 9°C otherwise 22°C).
material
pH
solution
initial final dissolution rate
(mole Si-g-l.s-1) albite albite albite albite albite albite
4.5 4.5 4.5 4.5 6.0 6.0
deion, water deion, water 2nd run citric acid pyrocatechol viol. citric acid pyrocatechol viol.
1.5-10 6.0-10 6.4-10 1.4-10 9.9-10 5.0-10
11 13 12 11 12 13
5.2-10 3.1-10 7.5-10 4.8-10 5.5-10 3.9-10
13 13 13 13 13 13
natural sand natural sand natural sand
4.5 6.0 4.5
deion, water (*) deion, water (*) fulvic acid (*)
4.5-10 12 1.6-10 12 9.9-10 12
1.2-10 t3 1.4-10 13 3.8-10 13
quartz
4.5
deion.water
3.9-10 12
3.1-10 13
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Alx en Provence, France.
balances of the Segeberger Forst considering the simplified assumptions in the evaluation and the uncertainties in measurement (DAHMKE, 1988, PETERSEN, 1988). The results from column experiments with organic ligands support the field observations that between pH 4 and 8 the dissolution behaviour and maximum solubility of aluminium in natural systems depends mainly on the physicochemical properties of the organic ligands. REFERENCES
ALBERTSEN, M., MATTHESS, G., PEKDEGER, A. & H.D. SCHULZ (1977) : Quantifizierung von Verwitterungsvorgaengen.- Geol. Rundschau, 69 : 532-545; Stuttgart.
313
DAHMKE, A. (1988) : Loesungskinetik von Feldspat-reichen Gesteinen und deren Bezug zu Verwitterung und Porenwasser-Chemie natuerlicher Sander-Sedimente.- Diss. Uni. Kiel : 168; Kiel. PETERSEN, A. (1988) : Laboruntersuchungen zum Einfluss organischer Komplexbildner auf die Kinetik der Feldspatverwitterung.- Diss. Uni. Kiel: 210; Kiel. SCHENK, D., PETERSEN, A. & G. MA'I~I'HESS (1989) : Acceleration and retardation of silicate weathering due to organic substances.- Proc. 6th int. Symp. on Water/RockInteraction, Malvern, LrK (MILES, ed.) : 605608; Rotterdam, Brooldield.